Thursday, April 30, 2009

Swine Influenza A/Mexico/2009 (H1N1) Update

29 April 2009 -- The situation continues to evolve rapidly. As of 18:00 GMT, 29 April 2009, nine countries have officially reported 148 cases of swine influenza A/H1N1 infection. The United States Government has reported 91 laboratory confirmed human cases, with one death. Mexico has reported 26 confirmed human cases of infection including seven deaths.

The following countries have reported laboratory confirmed cases with no deaths - Austria (1), Canada (13), Germany (3), Israel (2), New Zealand (3), Spain (4) and the United Kingdom (5).

Further information on the situation will be available on the WHO website on a regular basis.


Sequences of viral RNAs from 20 swine flu isolates have now been posted on the NCBI website. Included are isolates from California, Texas, New York, Ohio, Kansas, and Germany (taken from a tourist who returned from Mexico). It is difficult to understand why RNA sequences of none of the Mexican isolates have been posted, which would enable us to determine if the viruses in that country are different from the others. However, examination of the sequences of the New York and German isolates, which presumably originated in Mexico, reveal no significant differences with sequences from other isolates. From this information I conclude that the apparent higher virulence of swine flu in Mexico is not a consequence of a genetically diverged virus.

Other interesting information that can be gleaned from sequence information is contained in a statement from CDC: “…the HA, PB2, PB1, PA, NP, NS genes
contain gene segments from influenza viruses isolated from swine in North America [such as, A/swine/Indiana/P12439/00], while the NA and M genes are most closely related to corresponding genes from influenza viruses isolated in swine population in Eurasia.
However, the NA and M genes from 2 swine virus isolates from America are also closely related to the novel H1N1 virus (A/swine/Virginia/670/1987, A/swine/Virginia/67a/1987), if a reasonable nucleotide substitution rate is accepted. Thus, H1N1 from Mexico may be a swine flu virus strain of entirely American origin, possibly even of relatively ancient origin.” In the coming days I will attempt to construct a history of the evolution of swine influenza. In the meantime it may well be that this new human strain emerged from the US, as did the 1918-19 pandemic virus.

It is curious that CDC originally asserted that the new swine influenza virus inherited genes from human, pig, and bird viruses. Dr Anne Schuchat made this statement during a press conference on 23 Apr 2009, noting that “Preliminary testing of viruses from the 1st 2 patients shows that they are very similar. We know so far that the viruses contain genetic pieces from 4 different virus sources. This is unusual. The 1st is our North American swine influenza viruses. North American avian influenza viruses, human influenza viruses, and swine influenza viruses found in Asia and Europe. That particular genetic combination of swine influenza virus segments has not been recognized before in the US or elsewhere.”

I am not sure why the sequence information now available indicates a very different origin for these viruses.

Although many still describe this virus as swine flu, it is technically no longer a pig virus - having acquired the ability to be transmitted among humans and cause disease, it is now a human virus. I realize that the official strain names are cumbersome (A/Mexico/4482/2009 [H1N1]), and therefore it is likely that we will be using ’swine flu H1N1′ at least until the next pandemic.

No comments:

Post a Comment